As a technique for reducing the burden due to the repetition of blow-off and re-discharge of an ignition plug, suppressing unnecessary electric power consumption and continuing a spark discharge, the present applicant has devised an energy input circuit (not a publicly known art). The energy input circuit inputs electrical energy, after the start of an initial spark discharge (to be referred to as main ignition) by a well-known ignition circuit, to a battery voltage supply line from a low-voltage side of a primary coil before the main ignition is blown off; with the electrical energy input, the energy input circuit continuously applies electric current in the same direction to a secondary coil (DC secondary electric current), thereby continuing the spark discharge caused by the main ignition for an arbitrary time period (hereinafter, discharge continuation period). In addition, hereinafter, the spark discharge continued by the energy input circuit (the spark discharge following the main ignition) will be referred to as continuing spark discharge.
The energy input circuit controls, by controlling a primary electric current (input energy) in the discharge continuation period, the secondary electric current to sustain the spark discharge. By controlling the secondary electric current in the continuing spark discharge, it is possible to prevent blow-off of the ignition plug, reduce the burden of wear of electrodes, suppress unnecessary electric power consumption and continue the spark discharge.
Moreover, since the secondary electric current is applied in the same direction in the continuing spark discharge following the main ignition, it is difficult for the spark discharge to be interrupted in the continuing spark discharge following the main ignition. Therefore, with employment of the continuing spark discharge by the energy input, it is possible to prevent blow-off of the spark discharge even in an operating condition which is lean burn and in which a rotational flow is created in the cylinder.
Next, for the purpose of assisting the understanding of the present invention, a typical example of the energy input circuit (as described above, not a publicly known art), to which the present invention is not applied, will be described based on FIGS. 5-7. In addition, in FIG. 5, functional components identical to those in embodiments which will be described later are given the same reference signs as in the embodiments.
An ignition apparatus as shown in FIG. 5 includes a main ignition circuit 3 that causes the main ignition in a spark plug 1 by a full-transistor operation (on/off operation of an ignition switching means 13) and the energy input circuit 4 that performs the continuing spark discharge following the main ignition.
The energy input circuit 4 is configured with a boosting circuit 18 that boosts the voltage of an in-vehicle battery 11 (DC power source), an energy input switching means 27 for controlling the electrical energy inputted to the low-voltage side of the primary coil 7, and an energy input driver circuit 28 that controls the on/off operation of the energy input switching means 27.
FIG. 6 shows time charts illustrating the operation of the ignition apparatus in causing the main ignition.
The main ignition circuit 3 operates based on an ignition signal IGT provided by an ECU 5 (abbreviation of Engine Control Unit). Upon the ignition signal IGT being switched from low to high, the primary coil 7 of the ignition coil 2 is energized. Then, when the ignition signal IGT is switched from high to low and thus the energization of the primary coil 7 is interrupted, a high voltage is generated in the secondary coil 8 of the ignition coil 2, starting the main ignition in the ignition plug.
After the start of the main ignition in the ignition plug 1, the secondary electric current attenuates substantially in the shape of a sawtooth wave (see FIG. 6). In addition, in the time chart of the secondary electric current, the electric current value increases in the direction toward the negative side (downward in the figure).
FIG. 7 shows time charts illustrating the operation of the ignition apparatus in performing the continuing spark discharge after the main ignition.
The energy input circuit 4 operates based on a discharge continuation signal IGW and a secondary electric current command signal IGA provided by the ECU 5; the secondary electric current command signal IGA indicates a secondary electric current command value I2a. 
After the main ignition, for inputting energy to the secondary coil 8 before the secondary electric current drops to a “predetermined lower limit electric current value” (electric current value for sustaining the spark discharge) and thereby sustaining the spark discharge, the ECU 5 outputs both the discharge continuation signal IGW and the secondary electric current command signal IGA to the energy input circuit 4.
Upon the discharge continuation signal IGW being switched from low to high, the input of electrical energy from the low-voltage side of the primary coil 7 to the positive side is started. Specifically, during a time period in which IGW is high, by on/off controlling the energy input switching means 27, the secondary electric current is controlled so as to be kept at the secondary electric current command value I2a (see FIG. 7).
(Problematic Issue)
With employment of the continuing spark discharge by the energy input, it becomes difficult for blow-off of a spark discharge to occur even in an operating condition which is lean burn and in which a rotational flow is created in the cylinder.
In the ignition apparatus that is capable of performing the continuing spark discharge by the energy input, there are cases where only the main ignition is performed in an operating condition in which it is relatively difficult for blow-off to occur. That is, there are cases where: a predetermined operating condition, which is set according to the engine rotational speed, the engine load and the like, is defined as a main ignition region; and in the main ignition region, only the main ignition is performed. However, even in the region which is set as the operating condition where it is difficult for blow-off to occur, there is still a risk of blow-off occurring during the main ignition due to differences between individual engines, variation among cylinders and age deterioration.
Therefore, even in the ignition apparatus that is capable of performing the continuing spark discharge by the energy input, it is still necessary to take measures to determine blow-off in the main ignition region and thereby prevent a misfire.
In addition, as a technique for preventing blow-off in an ignition apparatus, there is disclosed in Patent Document 1 a technique of switching from a lean operation to a stoichiometric operation when it is impossible to secure a discharge time longer than or equal to a predetermined time. However, even in the stoichiometric operation, there are still cases where it is impossible to secure the discharge time due to differences between individual engines, variation among cylinders and age deterioration. Therefore, even if switched to the stoichiometric operation, there is still a risk that blow-off may occur, thereby resulting in a misfire.
Moreover, in Patent Document 2, there is disclosed detection of blow-off. However, according to the technique of Patent Document 2, a discharge is inhibited upon detection of blow-off. Therefore, there is a risk of resulting in a misfire.